Resistive memory elements include an element body which has resistive memory characteristics, and this element body exhibits, for example, a relatively high resistance in an initial state. When a voltage not less than a predetermined value is applied to the element body, the element body undergoes a change to a low resistance state, and this low resistance state is held (stored) even when the voltage is eliminated. On the other hand, when a voltage not less than a predetermined value is applied in the reverse direction to the element body in the low resistance state, the element body is returned to the high resistance state, and the high resistance state is held (stored) even when the voltage is eliminated.
This type of resistive memory element can be switched between a low resistance state and a high resistance state by applying a voltage not less than a threshold value in each of a forward direction and a reverse direction, and this switching allows the resistance to be changed and stored. The use of the resistive switching characteristics allows the resistive memory elements to be used not only as so-called resistive memory elements but also as switching elements.
It is believed that the resistive memory elements undergo a change in how electrons are likely to get over a barrier, by a change in the thickness of a depletion layer of a Schottky barrier, or by trapping or emission of electrons in or from the interface between an electrode and the element body made of a semiconductor and in or from a bulk level, thereby changing to the high resistance state and the low resistance state.
In the case of the resistive memory elements, there is a need to control the resistance with different voltages in polarity (referred to as bipolar), and basically, the resistive memory elements undergo switching from the high resistance state to the low resistance state when the voltage is applied to the forward side of a Schottky barrier, and switching from the low resistance state to the high resistance state when the voltage is applied to the reverse side thereof. The resistive memory elements are believed to develop the characteristics over the interface of the electrode, and supposed to be excellent in stability.
However, one of the problems is associated with resistance holding characteristics, and the resistive memory elements have the problem of poor stability particularly in the low resistance state, and switching the resistance to the high resistance state with an increase in temperature or with time, possibly because electrons are trapped in or emitted from the interface and the bulk level by the resistive switching. The techniques which can solve this problem include, for example, a technique described in Japanese Patent Application Laid-Open No. 2006-324447 (Patent Document 1).
Patent Document 1 proposes a technique for improving resistive memory characteristics. More specifically, Patent Document 1 adopts a Pt/Nb:SrTiO3/insulating film/electrode structure in a resistive memory element which has a structure with an oxide semiconductor (for example, Nb:SrTiO3) sandwiched by a first electrode (for example, a Pt electrode) which can form a Schottky barrier and another second electrode. In this case, the insulating film mentioned above is allowed to function as a barrier for suppressing electron emissions from interface traps in the oxide semiconductor, so that the probability of trapping or emitting electrons from the interface is reduced to result in an improvement in data holding characteristics (resistive memory effect).
However, in the case of the technique described in Patent Document 1, adverse effects such as a decrease in rate of resistance change and an increase in switching voltage are expected due to the introduction of the insulating layer.
Therefore, there is a need for a resistive memory element which provides a high rate of resistance change comparable to conventional cases and provides an excellent resistive memory effect without providing the insulating layer or the like as described above.    Patent Document 1: Japanese Patent Application Laid-Open No. 2006-324447